A powertrain with an electrically-variable transmission (EVT) typically has an input member, an output member, and two electric motor/generators connected to different members of planetary gear sets. Selectively engageable torque-transmitting mechanisms allow one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery-powered) mode. EVTs may improve vehicle fuel economy in a variety of ways, primarily by using one or both of the motor/generators for vehicle braking and using the regenerated energy to power the vehicle electrically, with the engine off. The engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Braking energy captured via regenerative braking (or electrical energy generated during periods when the engine is operating) is utilized during these engine-off periods. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on modes, allowing for a smaller engine without reducing vehicle performance. Additionally, the electrically-variable modes may allow the engine to be operated at or near the optimal efficiency point for a given power demand.
Various hybrid and electric powertrain architectures are known for managing the input and output torques of various prime movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Some EVTs provide for continuously variable speed ratios by combining features from both series and parallel hybrid power train architectures. EVTs are operable in fixed gears with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque, continuously variable speed ratios, electrically-dominated launches, regenerative braking and engine off idling.
Some hybrid vehicles utilize larger battery packs, adding to the vehicle weight, increasing fuel consumption and vehicle cost. These vehicles operate in an electric-only mode over a predetermined driving range, and then the engine charges the battery through one of the motor/generators motor when driving beyond the optimal range. Range, endurance and durability aspects suggest that a lower battery state of charge and higher fuel consumption required in charging the battery is generally undesirable. The rate of discharge of battery and engine power are calculated based on driver commanded throttle and vehicle speed